I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method for registration of a mobile communications device in a cellular communications environment.
II. Description of the Related Art
In a cellular communication system registration is the process a mobile station uses to notify a cellular communication system whether it is on the air and which cell it is receiving. The mobile station may be of a cellular radiotelephone or personal communication device typically in the form of a vehicle mounted unit or a hand carried portable unit. For calls that are directed towards a mobile station, the cellular system uses the registration information to reduce the amount of paging by determining whether to page a mobile station and, if so, to determine the set of cells to broadcast the page.
For calls directed to a mobile station, often called mobile terminated calls, the land system must determine whether the mobile station is powered on and the cell that the mobile station is receiving. To find the mobile station, the cellular system broadcasts a message, often called a page, in many cells. If the mobile station responds, the cellular system continues handling the call with further communication being directed to the cell containing the mobile station. It should be further understood that the paging process is used to locate the mobile station for any other transaction to be done with the mobile station.
If the system has no knowledge of the location of the mobile station, then the system must broadcast pages in every sector of every cell. As the amount of mobile terminated traffic increases, the communications resources to support system wide paging quickly becomes enormous in most large metropolitan areas.
A mobile station uses a process called registration to inform the cellular system where the mobile station is located. Several systems, such as AMPS and GSM, use a periodic or counter method to determine the location of the mobile station. The counter method is similar to the timer method described herein. Although these methods are far from ideal, they nevertheless can significantly reduce the amount of required paging.
An analysis of the timer method assumes the uniform hexagonal tessellation of the plane or uniform circular cells with radius r.sub.c. The same expected number of mobile stations, given by N.sub.a, is assumed to be in every cell.
In a cellular system implementing the timer method each mobile station registers every T.sub.r seconds. This requirement can be easily implemented by having the mobile station increment a counter periodically, or increment a counter in response to a global command from the system. By either supplying the maximum value of the counter or varying the increment rate, the system can vary T.sub.r. The average registration message rate, .lambda..sub.reg, per cell is thus given by the following equation: ##EQU1##
For a mobile terminated call, the system needs to determine the set of cells that the mobile station may have entered. If the mobile station can move at some maximum velocity v.sub.m, then the distance that the mobile station could have traveled is v.sub.m (t-t.sub.r) where t.sub.r is the time when the mobile station last registered. Unless the system knows where in the cell the mobile station was located when it registered, the system must assume that the mobile station was on the cell boundary. Unless the system has some direction information, it must assume that the mobile station was moving outward.
For a randomly chosen mobile station, the expected number of cells that must be paged can readily be found. The elapsed time since registration is a uniformly distributed random variable on [0, T.sub.r ]. Rather than considering the exact result which is discontinuous in the distance, more insight can be obtained by considering a quadratic approximation to the number of cells as function of the distance. The expected number of cells that the mobile station must be paged in, M.sub.s, is in accordance with the equation: ##EQU2##
The quantity r.sub.c /v.sub.m is the time that a mobile station moving at velocity v.sub.m takes move from the center of a cell to its boundary. If the excess outbound message rate is defined as the expected number of pages other than the answered page plus the expected number of messages that must be sent to acknowledge registrations, then the excess outbound message rate, .lambda..sub.ex, is determined by the equation: ##EQU3## where: M.sub.p is the number of times that a page message is repeated;
p.sub.p is the probability that the page is answered by the mobile station on a particular page repetition; and PA1 N.sub.a .lambda..sub.m .alpha..sub.t is the origination rate for mobile terminated calls in a cell.
As the interval between registrations, T.sub.r, decreases, the number of cells that must be paged decreases, but the acknowledgement rate increases. Thus some value of T.sub.r minimizes .lambda..sub.ex.
The main problem with the timer method is that paging must be done in area commensurate with the maximum vehicle velocity. If the region has a few routes that allow high velocity, then the system must use the highest velocity route for determining where to page. Portable units, which normally don't move very fast, may nevertheless be in a fast moving vehicle and cannot be counted as a separate class. Techniques which begin paging in a small region and then expand the paging region if the mobile station does not respond can be used to reduce the amount of paging at the expense of delay.
Another registration technique known as the zone method is also used to reduce the amount of paging in the cellular system. The zone method may simply divide the system into regions called zones. Thus, cells are grouped together to form fixed paging zones. Upon registration in a zone the mobile station is paged at all cell within the zone. The mobile station typically maintains a list of zones that it has recently visited. If the mobile station enters a zone not on the list, it then registers. Therefore as the mobile station travels throughout the system, each time it travels into a new zone it registers.
A variation of the basic zone method is described in the article entitled "A New Location Updating Method for Digital Cellular Systems," by Okasaka, Sadaatsu, Onoe, Seizo, Yasuda, Syuji, and Maebara, Akihiro, Proceedings of the 41st IEEE Vehicular Technology Conference, St. Louis, Mo., May 19-22, 1991, pp. 345-350. In this variation of the zone method, layers of zones are created along with the group of mobile stations divided by parameters, such as mobile station serial number, into which layer of zones the mobile station will register.
An analysis of the zone method again assumes the uniform hexagonal tesselation of the plane or uniform circular cells with radius r.sub.c. The same expected number of mobile stations, again given by N.sub.a, is assumed to be in every cell.
As mentioned above, in the zone method every cell in a system is assigned to a specific fixed zone. Every cell broadcasts the zone to which it is assigned. The mobile station keeps a list of zones that it has recently visited. Whenever a mobile station enters a zone not on its list, the mobile station registers and adds the zone to the list.
Vehicular traffic theory can be used to estimate the expected peak registration rate for a perimeter cell. If the cell is dominated by one or two main roadways, the rate is relatively easy to compute; otherwise, the computation can be quite tedious. A well known result is that the maximum capacity per lane of traffic on a well designed freeway is about 2000 vehicles per hour and occurs with vehicles moving about 50 km/hour. Good rule of thumb adjustments have been developed for other roadways. If the fraction of vehicles equipped with cellular telephones is known, then the expected peak registration rate can be obtained. For example, if 25% of the vehicles traveling on an 8 lane freeway were equipped with cellular telephones, then the expected peak registration rate for a perimeter cell would be 0.56 registrations per second.
One shortcoming of the zone method arises in the case of where a heavily travelled route, such as a freeway, intersects a zone boundary. In this instance all mobile stations register in the cells on the zone boundary through which the freeway passes. This situation can place a severe loading on the resources of these particular zone boundary located cell. One attempt to resolve this problem is to create a staggered or layered zone arrangement as mentioned above. In the fixed staggered zone arrangement, overlapping zones are created in which the particular zone a mobile station registers in is also a function of a serial or identification number of the mobile station. Such a scheme adds a further level of complexity and may not adequately distribute registrations amongst the cells.
Although the zone method does provide an improvement over timer or counter registration methods in that there is a reduction in cell pagings, there is still a need to reduce pagings even further.
It is therefore an object of the present invention to provide in a cellular communication system a method by which mobile station pagings may be reduced.
It is yet another object of the present invention to provide in a cellular communication system a mobile station registration scheme which provides an improvement in the distribution of mobile station registrations amongst the cells.